A generally used positive working photoresist composition comprises an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive substance. As examples of such a composition, the combinations of novolak-type phenol resins with naphthoquinonediazido-substituted compounds are described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. Further, as the most typical compositions, the combinations of novolak resins prepared from cresols and formaldehyde with trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonic acid esters are described in L. F. Thompson, Introduction to Microlithography, No. 2, 19, pages 112-121, ACS publisher.
In such a positive photoresist, which is basically constituted of a novolak resin and a quinonediazide compound, the novolak resin acts so as to provide high resistance to plasma etching and the naphthoquinonediazide compound functions as a dissolution inhibitor. Further, the naphthoquinonediazide has a characteristic of generating a carboxylic acid upon irradiation with light to lose the ability to inhibit dissolution, thereby heightening the solubility of the novolak resin in alkali.
From the aforementioned points of view, a large number of positive photoresist compositions comprising novolak resins and photosensitive compounds of naphthoquinonediazide type have hitherto been developed and put to practical use. In the working for reproducing lines having a width of the order of 0.8-2 .mu.m, those compositions have accomplished satisfactory results.
On the other hand, the integration degree of an integrated circuit has become higher and higher in recent years, so that the working for super fine patterns having a line width of half micron or below has come to be required in the production of semiconductor substrates for VLSI and the like.
As a means for elevating the fineness of patterns, it is known to shorten the wavelengths of an exposure light source used for the formation of resist patterns. Such a means can be explained by the Rayleigh equation relating to the resolution (line width) R of an optical system: EQU R=k.multidot..lambda./NA
wherein .lambda. is the wavelength of an exposure light source, NA is the aperture number of a lens and k is a process constant. From this equation, it can be understand that the attainment of high resolution, namely the diminution in the value of R, becomes possible by shortening the wavelength (.lambda.) of an exposure light source.
For instance, up to now i-line (365 nm) of a high-pressure mercury lamp has been used as a light source in the production of DRAM having an integration degree of 64 Mbit or below. In the mass production process of 256 Mbit DRAM, the use of KrF excimer laser (248 nm) instead of i-line as an exposure light source has been studied.
Further, light sources emitting light of shorter |wavelengths are under examination with the intention of producing DRAM with an integration degree of 1 Gbit or above, and the utilization of ArF excimer laser (193 nm), F.sub.2 excimer laser (157 nm), X-ray, electron beams or the like is considered effective (a book written by Koh Ueno et al., entitled "Photoresist Materials Responsive to Radiations of Short Wavelengths--Microlithography for Production of VLSI", published by Bunshin Shuppan, in 1988).
In a case where a conventional resist comprising a novolak resin and a naphthoquinonediazide compound is used for forming patterns by the lithography using far ultraviolet light or an excimer laser beam, it is difficult for the light to reach the lower part of the resist because a novolak resin and naphthoquinonediazide have strong absorption in the far ultraviolet region, so that such a resist has low sensitivity and can merely provide a pattern profile having a tapered shape.
As one means for solving the aforementioned problems, the chemical amplification type resist compositions disclosed, e.g., in U.S. Pat. No. 4,491,628 and European Patent 0,249,139 can be employed. The positive resist compositions of chemical amplification type are pattern forming materials of the type which generate acids in the exposed area by irradiation with an actinic radiation, such as far ultraviolet light, to cause the reaction utilizing these acids as catalyst, thereby making a difference of solubility in a developer between the areas -irradiated and unirradiated with the actinic radiation. By virtue of this solubility difference, a pattern can be formed on a substrate coated with a material of the foregoing type.
As examples of such a chemical amplification type resist composition, mention may be made of the combination of a compound capable of generating an acid by photolysis (hereinafter referred to as a photo-acid generator) with acetal or an O, N-acetal compound (JP-A-48-89003, wherein the term "JP-A" means an unexamined published Japanese patent application"), the combination of a photo-acid generator with an orthoester or amidoacetal compound (JP-A-51-120714), the combination of a photo-acid generator with a polymer having acetal or ketal groups in its main chain (JP-A-53-133429), the combination of a photo-acid generator with an enol ether compound (JP-A-55-12995), the combination of a photo-acid generator with an N-acyliminocarbonic acid compound (JP-A-55-126236), the combination of a photo-acid generator with a polymer having orthoester groups in its main chain (JP-A-56-17345), the combination of a photo-acid generator with a tertiary alkyl ester compound (JP-A-60-3625), the combination of a photo-acid generator with a silyl ester compound (JP-A-60-10247) and the combinations with a photo-acid generator with silyl ether compounds (JP-A-60-37549 and JP-A-60-121446). Those combinations have a quantum yield greater than 1 in principle, so that they exhibit high sensitivity.
As examples of a similar system to the above, which decomposes upon heating in the presence of an acid to acquire solubility in an alkali, mention may be made of the systems wherein tertiary or secondary carbon-containing (e.g., t-butyl, 2-cyclohexenyl) esters or carbonate compounds are combined with compounds capable of generating acids upon exposure to light, as described, e.g., in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, JP-A-5-181279, Polym. Eng. Sce., vol. 23, p. 1012 (1983); ACS. Sym., vol. 242, p. 11 (1984); Semiconductor World, the November number, p. 91 (1987); Macromolecules, vol. 21, p. 1475 (1988); and SPIE, vol. 920, p. 42 (1988); the systems wherein the acetal compounds described, e.g., in JP-A-4-219757, JP-A-5-249682 and JP-A-6-65332 are combined with the photo-acid generators as recited above, and the systems wherein the t-butyl ether compounds described, e.g., in JP-A-4-211258 and JP-A-6-65333 are combined with the photo-acid generators as recited above.
Those systems each use as a main component a resin whose basic skeleton is a poly(hydroxystyrene) having a small absorption in the wave length region around 248 nm. When KrF excimer laser is employed as an exposure light source, therefore, they can have high sensitivity and high resolution and form good patterns, namely they can be good systems, compared with a conventional naphthoquinonediazide/novolak resin system.
In cases where light sources emitting light of further short wavelengths, such as ArF excimer laser (193 nm) are used for exposure, even the chemical amplification systems recited above are unsuitable because the compounds having aromatic groups show essentially great absorption in the wave length region around 193 nm. Also, the utilization of poly(meth)acrylate, which shows small absorption in the wave length region around 193 nm, is described in J. Vac. Sci. Technol., B9, 3357 (1991). However, this polymer has a problem of having low resistance to dry etching which is generally carried out in a process of producing semiconductors, compared with conventional phenol resins having aromatic groups.
On the other hand, the fact that alicyclic group-containing polymers have dry etching resistance comparable to those of polymers having an aromatic group and show small absorption in the wave length region around 193 nm was reported in Proc. of SPIE, 1672, 66 (1992), and since then the utilization of those polymers has been examined extensively. Examples of such an alicyclic group-containing polymer include the polymers described in JP-A-4-39665, JP-A-5-80515, JP-A-5-265212, JP-A-5-297591, JP-A-5-346668, JP-A-6-289615, JP-A-6-324494, JP-A-7-49568, JP-A-7-185046, JP-A-7-191463, JP-A-7-199467, JP-A-7-234511, JP-A-7-252324 and JP-A-8-259626.
Further, it is shown in Proc. of SPIE, 1672, 66 (1992) that adamantyl groups have the best dry etching resistance of the alicyclic groups recited in those references, and the polymers having adamantyl groups are described in JP-A-4-39665, JP-A-7-199467, JP-A-7-234511, and JP-A-8-259626.
However, since these polymers are (meth)acrylate polymers, they do not have highly satisfactory dry etching resistance necessarily. Further, in copolymerizing a hydrophobic monomer having an adamantyl group with an acid-decomposable monomer required for image formation and a hydrophilic (polar) monomer required for adhesion to a substrate, those monomers were hard to introduce uniformly into the polymer chain. Accordingly, it occurred sometimes that the ununiform copolymerization resulted in a lowering of the solubility of the copolymer in a solvent or the reproducibility of the synthesis was poor.